Heterojunction semiconductor devices typically incorporate a junction between two materials with different band gaps, e.g., a heterojunction, as the channel instead of a doped region. Such devices use high mobility electrons generated by a heterojunction comprised of a highly-doped wider-bandgap n-type donor-supply layer, or unintentionally doped Aluminum-Gallium-Nitride (AlGaN), for example, and a non-doped narrower-bandgap layer with little or no intentional dopants, e.g., Gallium-Nitride (GaN).
In the framework of AlGaN/GaN heterostructures, there is often no dopant required in the AlGaN layer due to the strong spontaneous and piezoelectric polarization effect in such systems. For example, electrons from surface donors can be swept into the GaN channel by the intrinsic polarization induced electric field. In this instance, the electrons can move quickly without colliding with any impurities, due to the unintentionally doped (e.g., not intentionally doped) layer's relative lack of impurities or dopants, from which the electrons cannot escape.
The net result of such a heterojunction is to create a very thin layer of highly mobile conducting electrons with very high concentration or density, giving the channel very low resistivity. This layer is known as a two-dimensional electron gas (2DEG). This effect for instance can be utilized in a field-effect transistor (FET), where the voltage applied to the Schottky gate alters the conductivity of this layer to form transistor structures.
One kind of such a transistor is a high-electron mobility transistor (HEMT) including Gallium Nitride is known as an Aluminum Gallium Nitride/Gallium Nitride (AlGaN/GaN) HEMT, or an AlGaN/GaN HEMT. Typically, AlGaN/GaN HEMTs can be fabricated by growing crystalline films of GaN, AlGaN, etc. on a substrate, e.g., sapphire, silicon (Si)(111), silicon carbide (SiC) and so on, through an epitaxial crystal growth method, e.g., metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and so on, and processing the epitaxial substrate thus grown, to form the desired structures.
Recently, AlGaN/GaN HEMTs and Schottky diodes have received attention for their ability to operate at high voltage and high current levels, which results in enhanced high-power performance, as a benefit of the inherent high-density 2DEG, high electron mobility, and high critical breakdown electric field. As a consequence, the wide bandgap AlGaN/GaN HEMTs are emerging as excellent candidates for radio-frequency (RF) and microwave power amplifiers.
In some devices, e.g. normally-on or normally-off devices, the semiconductor device is switched between the off-state, in which the 2DEG is disrupted under the Schottky gate, and an on-state in which a high current is produced at a low voltage. The design of such devices typically targets a trade-off between power losses in the on-state, off-state and during switching.
One of the problems with such devices is a high leakage current during the off-state. Such a leakage current is determined by the potential barrier for electrons between the Schottky metal and the 2DEG. This problem has been addressed in US 2010/0084687 A1, where a fluorine-doped enhanced back barrier is provided underneath the Schottky gate. This however has the drawback of negatively influencing the on-characteristics of the device.